Skipping frequencies for variable speed controls

ABSTRACT

A control for an electric motor is utilized to avoid operation in or near the resonance frequencies for the electric motor and its associated system components. The resonance frequencies can be identified experimentally at the design stage, or during operation of a component and electric motor. During start-up, shutdown or frequency adjustment, the control drives the speed through the resonance frequency zones more rapidly, and also avoids operation in or near those resonance frequencies during steady state operation. In disclosed embodiments, the electric motors are associated with fans, pumps and compressors in a refrigerant system.

BACKGROUND OF THE INVENTION

This invention relates to a method of avoiding objectionable frequenciesfor equipment driven by a variable speed motor, and in particular formotors driving equipment utilized in refrigerant systems.

Electric motors are utilized in refrigerant systems to drive the fans,pumps and compressors. As is known, in a basic refrigerant system, acompressor compresses a refrigerant and delivers that refrigerantdownstream to a first heat exchanger. The first heat exchanger exchangesheat between the refrigerant and another heat transfer media such asair, and passes the refrigerant to an expansion device. From theexpansion device, the refrigerant is delivered to another heatexchanger, and heat is again exchanged with another heat transfer media.From the second heat exchanger, refrigerant is returned to thecompressor. Fans or pumps are associated with each of the two heatexchangers, and a motor is typically associated with each fan or pump.Further, a motor is provided to drive a compressor unit. Also,refrigerant system circuits can have other components such as forexample fans or pumps driven by a variable speed motors.

Variable speed motors are becoming more widely utilized in refrigerantsystems. A variable speed motor provides a designer with enhancedflexibility in system operation and control. For instance, the capacityof the refrigerant system can be changed by varying the speed of thecompressor motor. Thus, variable speed motors and driven equipment canoperate across a variety of operational frequencies. Typically, thevariable speed motor starts from a frequency of zero and is ramped uptoward a desired operational frequency. Thus, the frequency advancesfrom zero upwardly to an operational frequency, which may be selected toachieve a desired cooling capacity, etc. Further, at shutdown, thefrequency decreases from that operational frequency back towards zero.

A control for the variable speed motor may change the operationalfrequency, as conditions or load demands faced by the refrigerant systemchange.

One problem with the above-described systems is that for any mechanicalsystems, there are certain frequencies, which have undesirable aspects,for example, as caused by either acoustic or mechanical resonances. Suchfrequencies could cause excessive vibration and internal pulsationsresulting in component damage as well as undesirable noise potentiallyleading to customer discomfort. The above-described systems, with themotor frequencies starting from zero and advancing upwardly towards thedesired operational frequency, may pass through these resonancefrequencies both at start-up and at shutdown. Also, as the controlchanges frequencies during operation to satisfy external load demands,it may sometimes move the electric motor operation to one of theresonance frequency zones that should be avoided. The system resonancefrequencies can also be excited by multiples of motor running speedfrequencies, or by the running frequencies (or their multiples) of thedriven equipment itself. It should be pointed out that the equipmentrunning speed frequency can be different than that of the motor, if forexample the driven equipment is attached to the motor via a gearbox.

This is undesirable, as excessive vibration, noise and pulsations mayoccur and result in damage of the system components.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, the undesirable frequenciesfor a particular component associated with an electric motor areidentified. As motor frequency is varied, the control is programmed toavoid those undesirable frequency zones. In known control algorithms foran electric motor associated with a refrigerant system, the frequency isvaried, and the resultant change from the refrigerant system operationis monitored. The control has a desired system operational feature. Thatdesired operational feature may be the cooling capacity of therefrigerant system, as an example. In one well-known control method, thecontrol does not necessarily determine the required operationalfrequency of the motor. Instead, the control varies the operationalfrequency and monitors the resultant change on the refrigerant systemuntil a frequency is found at which the operation of the system is asdesired. Typically, the frequency is varied in incremental steps. Withthis invention, the control will vary the operational frequency of theelectric motor, but will skip operation in zones associated with theundesirable frequencies.

As mentioned above, the disclosed application for such a control andmethod would be for the fans, pumps and compressors driven by a motor ina refrigerant system. However, other system components may benefitfrom-this basic control concept.

The undesirable frequencies (frequency that would normally be associatedwith either acoustical or mechanical resonances) may be determinedexperimentally, in a laboratory for a particular type of equipment, ormay be determined by various types of sensors mounted upon thecomponent. As an example, sensors can be mounted on a fan housing, andsense one of the vibration characteristics. The frequency of the motoror the running frequency of the driven equipment or multiples thereofcan be associated with the varying vibration level, and in this manner,the frequencies most subject to vibration and exceeding the desiredlevel can be identified, and then avoided, or associated with a “higherslope” of ramp-up during the start-up, shutdown and frequency adjustmentprocesses. The same reasoning would apply to measurement of excessivepulsations, as for example measured by dynamic pressure transducersinstalled into the piping adjacent to the system components.

Additionally, the system may self-learn during operation by comparing,for instance, vibration sensor measurements to acceptable values and thecontroller may include frequencies to be avoided to the skip frequencylist in a stored database.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a refrigerant system incorporating thepresent invention.

FIG. 2 is a graph of one of the vibration characteristics versus theoperational frequency of an electric motor.

FIG. 3A is a graph of the operational frequency over time in accordancewith an inventive method.

FIG. 3B is a flowchart of the inventive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a refrigerant system 20 incorporating compressor 22delivering a compressed refrigerant to a heat exchanger 24. The heatexchanger 24 is associated with a fan 26 for driving air over the heatexchanger 24. The fan 26 is associated with a motor, as known. Avariable speed control C and a transducer T are associated with the fan26. The variable speed control C drives the motor for the fan 26, andthe transducer T may identify one of the parameters associated withvibration level at the fan.

Refrigerant passes from the heat exchanger 24 downstream to an expansiondevice 28, and then to another heat exchanger 30. The heat exchanger 30is associated with its own fan 32. A variable speed motor control C andtransducer T are also associated with the fan 32.

The refrigerant passes from the heat exchanger 30 back to the compressor22. As is known, a motor drives a compressor unit 22, and a variablespeed control C and a transducer T are associated with the compressor22.

While refrigerant systems such as are utilized for air conditioningtypically have fans moving air over the heat exchangers, otherrefrigerant systems may be utilized with fluids other than air. As anexample, the assignee of the present invention has recently developed asystem wherein a refrigerant system is utilized to heat water. In such acase, at least one of the heat exchangers would include a pump movingwater over the heat exchanger, rather than a fan moving air. The presentinvention would extend to such systems.

As shown in FIG. 2, if one were to plot the operational frequency of amotor versus one of the characteristics associated with the vibration,pulsation or sound level within a component associated with the motor,there would be typically one or more “resonance frequencies” at whichthe vibration/pulsation/sound level increases dramatically. As shown inFIG. 2, these frequency zones are designated as X₁ and X₂. The presentinvention seeks to limit the operation of the motors in or near thesefrequencies, or to “skip” these frequencies.

FIG. 3A is a control diagram of the present invention. As shown, thecontrol may operate by moving through a series of incremental steps A,B, C, and D. The control moves to one of these steps, and operates therefrigerant system. The operation of the refrigerant system ismonitored, and if the refrigerant system is operating as desired, thecontrol will remain at that operational frequency. However, it istypical that the control must vary the operational frequency, and overtime certainly will often need to vary the operational frequency whenexternal load demands change or the indoor space is reaching the desiredconditions. As shown in FIG. 3A, when the operational frequency isvaried, it is varied in steps that avoid the resonance frequency zones.Thus, if the control starts the refrigerant system 20 operating at thefrequency A, and determines that the operation of the refrigerant system20 does not correspond to a desired state to satisfy coolingrequirements, it will advance to frequency B. Again, if frequency B doesnot provide the desired result, the control will increase frequency toC. From frequency C, a shorter incremental step to frequency D may beutilized. This is an overly simplified explanation of the controls,which may be known in the art (other than the inventive addition ofskipping through the zones X₁ and X₂). Typically, the incremental stepsmight be smaller, and/or of different size, and there may be severalbetween each of the resonance frequency zones. However, the FIG. 3A doesprovide an understanding of the operation basics.

In this manner, while the motor frequency will pass through both zonesX₁ and X₂ during start-up, shutdown or frequency adjustment, it willonly be in those zones for a brief period of time. Thus, the excessivevibration, noise or pulsation will not be felt for any undue length oftime.

Moreover, as the control C controls the speed of the motor duringoperation, the speed may be varied dependent on operational conditions.That is, a worker of ordinary skill in the art would recognize variousreasons for which variation in the speed may be desirable. As oneexample only, as the desired capacity for the compressor changes, itwould be desirable to vary the motor speed for the compressor andconsequently perhaps fan or pump speed as well. The controls C for thisinvention are programmed (as described below) to avoid operating in thezones X₁ and X₂, regardless of whether operation in such zones may bedictated by the operational conditions.

The zones X₁ and X₂ may be determined in any one of several manners. Inthe illustrated embodiment, the transducers T are utilized to find theundesirable frequencies (as mentioned earlier the undesirablefrequencies may be associated with system or component resonances butcan be “undesirable” for other considerations as well) by monitoring atleast one of vibration, pulsation, sound or other characteristics on theseveral system components. Alternatively, the resonance frequencies canbe determined experimentally for a specific family of components or typeof the equipment and then pre-programmed into the operating logic of thecontrollers C.

Another method would be to utilize a system that will “self-learn” thefrequencies to be avoided. Another method might be to vary the speedduring initial operation to “hunt” for the resonance frequencies to beavoided and then input these frequencies into the system controller suchthat they can be avoided. Such cases may surface when system naturalfrequencies are installation dependant or cannot be generalized for anentire product line. The ‘hunt’ for these undesirable frequencies may berepeated on a regular basis to detect whether there has been a change inthese resonance frequencies over time.

The transducer T can be an accelerometer, and can be mounted on the fanor compressor housing, on interconnecting pipes, on the heat exchangers,etc. Other types of transducers such as proximity sensors, velocitypick-up vibration sensors, etc. can be utilized as well. Further,pulsation/acoustic measurement transducers such as a dynamic pressuresensor as well as other types of sound measurements, which may be remoteto the component at issue, can be utilized. Furthermore, for redundancypurposes, multiple transducers can be employ to determine undesirableoperational frequency zones.

FIG. 3B is a flowchart of this invention, and shows the start-up orshutdown procedure, as well as the continuous operation while avoidingthe “skipped” frequencies.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. An electric motor and component comprising: an electric motor fordriving an associated component; and a control for said electric motor,said control being operable to drive said electric motor through avariable range of operational frequencies, said control storing at leastone undesirable zone of operational frequency for said electric motor,and said control limiting an amount of time said electric motor operatesin said at least one undesirable zone of operational frequency.
 2. Theelectric motor and component as set forth in claim 1, wherein saidcontrol moves between frequencies by varying frequency and monitoringoperation of the component, and said control not moving an operationalfrequency of said electric motor to said at least one undesirable zoneof operational frequency.
 3. The electric motor and component as setforth in claim 1, wherein said control avoids steady state operation insaid at least one undesirable zone of operational frequency duringoperation of said electric motor and said component.
 4. The electricmotor and component as set forth in claim 1, wherein said component isincluded in a refrigerant system.
 5. The electric motor and component asset forth in claim 1, wherein said control being programmed to moverapidly through said at least one undesirable zone of operationalfrequency during start-up of said motor.
 6. The electric motor andcomponent as set forth in claim 1, wherein said control being programmedto move rapidly through said at least one undesirable zone ofoperational frequency during shutdown of said electric motor.
 7. Arefrigerant system comprising: a compressor, said compressor beingprovided with a first electric motor for driving a compressor; a firstheat exchanger downstream of said compressor, and a first fluid-movingdevice associated with said first heat exchanger, said firstfluid-moving device being provided with a second electric motor; anexpansion device downstream of said first heat exchanger, a second heatexchanger downstream of said expansion device, and a second fluid-movingdevice associated with said second heat exchanger, said secondfluid-moving device being provided with a third electric motor; and avariable speed control for at least one of said first, second and thirdelectric motors, said control being programmed to store at least oneundesirable operational frequency zone, and said control beingprogrammed to minimize an amount of time said at least one of saidfirst, second and third electric motors operate in said undesirableoperational frequency zone.
 8. The refrigerant system as set forth inclaim 7, wherein at least one of said first and second fluid-movingdevices moves air over its associated heat exchanger.
 9. The refrigerantsystem as set forth in claim 7, wherein at least one of said first andsecond fluid-moving devices moves liquid through its associated heatexchanger.
 10. The refrigerant system as set forth in claim 7, whereinsaid control moves between frequencies by varying frequency andmonitoring operation of said refrigerant system, and said control notmoving an operational frequency of said at least one of said first,second and third electric motors to said undesirable operationalfrequency zone.
 11. The refrigerant system as set forth in claim 7,wherein said control avoids steady state operation at said undesirableoperational frequency during operation of said at least one of saidfirst, second and third electric motors.
 12. The refrigerant system asset forth in claim 7, wherein said control being programmed to moverapidly through said at least one undesirable zone of operationalfrequency during start-up of said motor.
 13. The refrigerant system asset forth in claim 7, wherein said control being programmed to moverapidly through said at least one undesirable zone of operationalfrequency during shutdown of said electric motor.
 14. The refrigerantsystem as set forth in claim 7, wherein said undesirable operationalfrequency zones are pre-determined.
 15. The refrigerant system as setforth in claim 7, wherein said undesirable operational frequency zonesare determined by placing transducers on a component or part of therefrigerant system and monitoring operation of said transducer as anoperational frequency of said at least one of said first, second andthird electric motors changes.
 16. The refrigerant system as set forthin claim 15, wherein said transducers include vibration transducers. 17.The refrigerant system as set forth in claim 15, wherein saidtransducers include pressure pulsation sensors.
 18. The refrigerantsystem as set forth in claim 15, wherein said transducers include soundtransducers.
 19. The refrigerant system as set forth in claim 7, whereinat least one of said first, second and third electric motors areprovided with a control programmed to store an undesirable operationalfrequency zone, and said controls each being programmed to minimize theamount of time each of said first, second and third electric motorsoperates in said undesirable operational frequency zone.
 20. A method ofoperating an electric motor comprising the steps of: (1) providing anelectric motor for driving a component, said electric motor beingoperable at a varying operational frequency, and said electric motorbeing provided with a control, said control storing at least one zone ofoperational frequency for said electric motor; and (2) utilizing saidcontrol to drive said electric motor, and minimize an amount of timesaid electric motor spends at said at least one zone of operationalfrequency.
 21. The method as set forth in claim 20, wherein said controlvaries an operational frequency, and monitors operation of saidcomponent, and said control not varying the operational frequency intosaid at least one zone of operational frequency.
 22. The method as setforth in claim 20, wherein said control further avoids operation at saidat least zone of operational frequency during steady state operation ofsaid electric motor.
 23. The method as set forth in claim 20, whereinsaid control avoids operation at said at least one zone of operationalfrequency during transient operation of said electric motor by quicklypassing through the at least one zone of operational frequency duringthe transient operation.
 24. The method as set forth in claim 20,wherein said component being a compressor in a refrigerant system. 25.The method as set forth in claim 20, wherein said component being afluid-moving device for moving a fluid over a heat exchanger in arefrigerant system.
 26. The method as set forth in claim 25, whereinsaid fluid-moving device is a pump for moving liquid through the heatexchanger.
 27. The method as set forth in claim 25, wherein saidfluid-moving device is a fan for moving air over the heat exchanger.